arXiv subjects

Krisztián Vida

Publications and source records attributed to Krisztián Vida.

34 records · Page 2Linked to original sources

arXiv2019

Deriving photospheric parameters and elemental abundances for a sample of stars showing the FIP effect

One puzzling question in solar physics is the difference between elemental abundances in the photosphere and the corona. Elements with low first ionization potential (FIP) can be overabundant in the corona compared to the photosphere under certain circumstances. The same phenomenon has been observed on a handful of stars, while a few of them show the inverse effect. But not all the stars in the original sample had precise photospheric abundances derived from optical spectra, so for some the solar values were adopted. In this work we make homogeneous abundance measurements from optical spectroscopy. We collected spectra of 16 stars showing the FIP effect with the 1-m RCC telescope of Konkoly Observatory, with resolution of $λ/ Δλ\sim 21\,000$. We determine the fundamental astrophysical parameters ($T_\mathrm{eff}$, $\log g$, $[M/H]$, $ξ_\mathrm{mic}$, $v \sin i$) and individual elemental abundances with the SME spectral synthesis code using MARCS2012 model atmosphere and spectral line parameters from the Vienna Atomic Line Database (VALD).

astro-ph.SR↗

arXiv2019

The Type II-P Supernova 2017eaw: from explosion to the nebular phase

The nearby SN 2017eaw is a Type II-P (``plateau') supernova showing early-time, moderate CSM interaction. We present a comprehensive study of this SN including the analysis of high-quality optical photometry and spectroscopy covering the very early epochs up to the nebular phase, as well as near-UV and near-infrared spectra, and early-time X-ray and radio data. The combined data of SNe 2017eaw and 2004et allow us to get an improved distance to the host galaxy, NGC 6946, as $D \sim 6.85$ $\pm 0.63$ Mpc; this fits in recent independent results on the distance of the host and disfavors the previously derived (30% shorter) distances based on SN 2004et. From modeling the nebular spectra and the quasi-bolometric light curve, we estimate the progenitor mass and some basic physical parameters for the explosion and the ejecta. Our results agree well with previous reports on a RSG progenitor star with a mass of $\sim15-16$ M$_\odot$. Our estimation on the pre-explosion mass-loss rate ($\dot{M} \sim3 \times 10^{-7} -$ $1\times 10^{-6} M_{\odot}$ yr$^{-1}$) agrees well with previous results based on the opacity of the dust shell enshrouding the progenitor, but it is orders of magnitude lower than previous estimates based on general light-curve modeling of Type II-P SNe. Combining late-time optical and mid-infrared data, a clear excess at 4.5 $μ$m can be seen, supporting the previous statements on the (moderate) dust formation in the vicinity of SN 2017eaw.

astro-ph.HE↗

arXiv2019

The quest for stellar coronal mass ejections in late-type stars: I. Investigating Balmer-line asymmetries of single stars in Virtual Observatory data

Flares and CMEs can have deleterious effects on their surroundings: they can erode atmospheres of orbiting planets over time and also have high importance in stellar evolution. Most of the CME detections in the literature are single events found serendipitously sparse for statistical investigation. We aimed to gather a large amount of spectral data of M-dwarfs to drastically increase the number of known events to make statistical analysis possible in order to study the properties of potential stellar CMEs. Using archive data we investigated asymmetric features of Balmer-lines, that could indicate the Doppler-signature of ejected material. Of more than 5500 spectra we found 478 with line asymmetries--including nine larger events--on 25 objects, with 1.2-19.6 events/day on objects with line asymmetries. Most events are connected with enhanced Balmer-line peaks, suggesting these are connected to flares similar to solar events. Detected speeds mostly do not reach surface escape velocity: the typical observed maximum velocities are in the order of 100-300km/s , while the typical masses of the ejecta were in the order of $10^{15}-10^{18}$g. Statistical analysis suggests that events are more frequent on cooler stars with stronger chromospheric activity. Detected maximum velocities are lower than those observed on the Sun, while event rates were somewhat lower than we could expect from the solar case. These findings may support the idea that most of the CMEs could be suppressed by strong magnetic field. Alternatively, it is possible that we can observe only an early low-coronal phase before CMEs are accelerated at higher altitudes. Our findings could indicate that later-type, active dwarfs could be a safer environment for exoplanetary systems CME-wise than previously thought, and atmosphere loss due to radiation effects would play a stronger role in exoplanetary atmosphere evolution than CMEs.

astro-ph.SR↗

arXiv2018

The Connection between Starspots and Flares on Main-Sequence Kepler Stars

Starspots and flares are indicators of stellar magnetic activity that can both be studied in greater detail by utilizing the long-term, space-based archive of the Kepler satellite. Here, we aim to investigate a subset of the Kepler archive to reveal a connection between the starspots and the stellar flares, in order to provide insight into the overall stellar magnetic field. We use the flare-finding algorithm FLATW'RM in conjunction with a new suite of algorithms that aim to locate the local minima caused by starspot groups. We compare the phase difference between the time of maximum flux of a flare and the time of minimum stellar flux due to a starspot group. The strongest flares do not appear to be correlated to the largest starspot group present, but are also not uniformly distributed in phase with respect to the starspot group. The weaker flares, however, do show an increased occurrence close to the starspot groups.

astro-ph.SR↗

arXiv2018

Finding flares in Kepler data using machine learning tools

Archives of long photometric surveys, like the Kepler database, are a gold mine for studying flares. However, identifying them is a complex task; while in the case of single-target observations it can be easily done manually by visual inspection, this is nearly impossible for years-long time series for several thousand targets. Although there exist automated methods for this task, several problems are difficult (or impossible) to overcome with traditional fitting and analysis approaches. We introduce a code for identifying and analyzing flares based on machine learning methods, which are intrinsically adept at handling such data sets. We used the RANSAC (RANdom SAmple Consensus) algorithm to model light curves, as it yields robust fits even in case of several outliers, like flares. The light curve is divided into search windows, approximately in the order of the stellar rotation period. This search window is shifted over the data set, and a voting system is used to keep false positives to a minimum: only those flare candidate points are kept that were identified in several windows as a flare. The code was tested on the K2 observations of the TRAPPIST-1, and on the long cadence data of KIC 1722506. The detected flare events and flare energies are consistent with earlier results from manual inspections.

astro-ph.SR↗

arXiv2017

Frequent flaring in the TRAPPIST-1 system - unsuited for life?

We analyze short cadence K2 light curve of the TRAPPIST-1 system. Fourier analysis of the data suggests $P_\mathrm{rot}=3.295\pm0.003$ days. The light curve shows several flares, of which we analyzed 42 events, these have integrated flare energies of $1.26\times10^{30}-1.24\times10^{33}$ ergs. Approximately 12% of the flares were complex, multi-peaked eruptions. The flaring and the possible rotational modulation shows no obvious correlation. The flaring activity of TRAPPIST-1 probably continuously alters the atmospheres of the orbiting exoplanets, making these less favorable for hosting life.

astro-ph.SR↗

arXiv2016

Hexapod Design For All-Sky Sidereal Tracking

In this paper we describe a hexapod-based telescope mount system intended to provide sidereal tracking for the Fly's Eye Camera project -- an upcoming moderate, 21"/pixel resolution all-sky survey. By exploiting such a kind of meter-sized telescope mount, we get a device which is both capable of compensating for the apparent rotation of the celestial sphere and the same design can be used independently from the actual geographical location. Our construction is the sole currently operating hexapod telescope mount performing dedicated optical imaging survey with a sub-arcsecond tracking precision.

astro-ph.IM↗

arXiv2015

Isotropic and anisotropic pointing models

This paper describes an alternative approach for generating pointing models for telescopes equipped with serial kinematics, esp. equatorial or alt-az mounts. Our model construction does not exploit any assumption for the underlying physical constraints of the mount, however, one can assign various effects to the respective components of the equations. In order to recover the pointing model parameters, classical linear least squares fitting procedures can be applied. This parameterization also lacks any kind of parametric singularity. We demonstrate the efficiency of this type of model on real measurements with meter-class telescopes where the results provide a root mean square accuracy of 1.5-2 arcseconds.

astro-ph.IM↗

arXiv2015

The January 2015 outburst of a red nova in M31

M31N 2015-01a (or M31LRN 2015) is a red nova that erupted in January 2015 -- the first event of this kind observed in M31 since 1988. Very few similar events have been confirmed as of 2015. Most of them are considered to be products of stellar mergers. Results of an extensive optical monitoring of the transient in the period January-March 2015 are presented. Eight optical telescopes were used for imaging. Spectra were obtained on BTA, GTC and the Rozhen 2m telescope. We present a highly accurate 70 d lightcurve and astrometry with a 0.05" uncertainty. The color indices reached a minimum 2-3 d before peak brightness and rapidly increased afterwards. The spectral type changed from F5I to F0I in 6 d before the maximum and then to K3I in the next 30 d. The luminosity of the transient was estimated to $8.7^{+3.3}_{-2.2}\times10^{5}L_{\odot}$ during the optical maximum. Both the photometric and the spectroscopic results confirm that the object is a red nova, similar to V838 Monocerotis.

astro-ph.SR↗

arXiv2014

The Fly's Eye Camera System -- an instrument design for large étendue time-domain survey

In this paper we briefly summarize the design concepts of the Fly's Eye Camera System, a proposed high resolution all-sky monitoring device which intends to perform high cadence time domain astronomy in multiple optical passbands while still accomplish a high étendue. Fundings have already been accepted by the Hungarian Academy of Sciences in order to design and build a Fly's Eye device unit. Beyond the technical details and the actual scientific goals, this paper also discusses the possibilities and yields of a network operation involving $\sim10$ sites distributed geographically in a nearly homogeneous manner. Currently, we expect to finalize the mount assembly -- that performs the sidereal tracking during the exposures -- until the end of 2012 and to have a working prototype with a reduced number of individual cameras sometimes in the spring or summer of 2013.

astro-ph.IM↗

arXiv2014

The Fly's Eye project: sidereal tracking on a hexapod mount

The driving objective of the Fly's Eye Project is a high resolution, high coverage time-domain survey in multiple optical passbands: our goal is to cover the entire visible sky above the 30 deg horizontal altitude with a cadence of 3 min. Imaging is intended to perform with 19 wide-field cameras mounted on a hexapod platform. The essence of the hexapod allows us to build an instrument that does not require any kind of precise alignment and, in addition, the similar mechanics can be involved independently from the geographical location of the device. Here we summarize our early results with a single camera, focusing on the sidereal tracking as it is performed with the hexapod built by our group.

astro-ph.IM↗

arXiv2013

Doppler imaging of the double-lined active binary V824 Ara

We introduce an iterative spectral disentangling technique combined with Doppler imaging in order to recover surface temperature maps for both components of double-lined active binary systems. Our method provides an opportunity to separate spectra of the active components while minimizing the unwanted disturbances on the given line profile from the other component. The efficiency of the method is demonstrated on real data of the double-lined RS CVn-type binary V824 Ara. The resulting Doppler images reveal cool spots on the polar regions as well as low-latitude features on both of the stars. Moreover, both components have hot spots, that are facing each other. This may indicate interconnection between the stellar magnetic fields.

astro-ph.SR↗

arXiv2013

Activity on a Li-rich giant: DI Psc revisited

We present a new Doppler imaging study for the Li-rich single K-giant DI Psc. Surface temperature maps are reconstructed for two subsequent rotation cycles. From the time evolution of the spot distribution antisolar-type differential rotation pattern is revealed. We show marks of non-uniform Li-abundance as well. The possible connection between the current evolutionary phase of the star and its magnetic activity is briefly discussed.

astro-ph.SR↗

arXiv2013

Surface differential rotation of IL Hya from time-series Doppler images

We present a time-series Doppler imaging study of the K-subgiant component in the RS CVn-type binary system IL Hya (P orb=12.905 d). From re-processing the unique long-term spectroscopic dataset of 70 days taken in 1996/97, we perform a thorough cross-correlation analysis to derive surface differential rotation. As a result we get solar-type differential rotation with a shear value alpha of 0.05, in agreement with preliminary suggestions from previous attempts. A possible surface pattern of meridional circulation is also detected.

astro-ph.SR↗

arXiv2013

On the reliability of measuring differential rotation of spotted stars

Cross-correlation of consecutive Doppler images is one of the most common techniques used to detect surface differential rotation (hereafter DR) on spotted stars. The disadvantage of a single cross-correlation is, however, that the expected DR pattern can be overwhelmed by sudden changes in the apparent spot configuration. Another way to reconstruct the image shear using Doppler imaging is to include a predefined latitude-dependent rotation law in the inversion code (`sheared image method'). However, special but not unusual spot distributions, such like a large polar cap or an equatorial belt (e.g., small random spots evenly distributed along the equator), can distort the rotation profile similarly as the DR does, consequently, yielding incorrect measure of the DR from the sheared image method. To avoid these problems, the technique of measuring DR from averaged cross-correlations using time-series Doppler images (`ACCORD') is introduced and the reliability of this tool is demonstrated on artificial data.

astro-ph.SR↗

arXiv2013

The Fly's Eye Camera System -- an instrument design for large étendue time-domain survey

In this paper we briefly summarize the design concepts of the Fly's Eye Camera System, a proposed high resolution all-sky monitoring device which intends to perform high cadence time domain astronomy in multiple optical passbands while still accomplish a high étendue. Fundings have already been accepted by the Hungarian Academy of Sciences in order to design and build a Fly's Eye device unit. Beyond the technical details and the actual scientific goals, this paper also discusses the possibilities and yields of a network operation involving ~10 sites distributed geographically in a nearly homogeneous manner. Currently, we expect to finalize the mount assembly -- that performs the sidereal tracking during the exposures -- until the end of 2012 and to have a working prototype with a reduced number of individual cameras sometimes in the spring or summer of 2013.

astro-ph.IM↗